It is the long-term objective of this investigation to follow the development of specific cell surface components, to determine their relationship to cellular differentiation, and to elucidate the factors which regulate their appearance or disappearance. In the first phase of this investigation, a number of cell surface receptors and secretory components in the rat intestinal epithelium were identified by the use of specific lectins. These cell surface components were found to be associated with cells at various stages of differentiation. The second phase of this investigation will continue work on the isolation, characterization, and developmental patterns of these receptors as well as begin an analysis of the factors controlling their appearance on the cell surface.